The blue whale – the largest living animal on the planet – can eat half a million calories in a single titanic gulp. It accelerates to high speeds and lunges into a swarm of tiny krill. It opens the world’s biggest mouth, which expands like a balloon to swallow up to 110 tonnes of water. The mouth closes, and the tongue pushes the water against bristly plates called baleen, which filter out the krill.

The blue whale’s skull is specially adapted to allow it to engulf large volumes of water. If you push against your chin, nothing happens because the two halves of your lower jaw are firmly fused together at the front. That’s not the case for the blue whale. The joint between its lower jaws – the mandibular symphysis – is particularly loose and elastic, allowing the jaw to flex and expand with the incoming tide of water and krill.

This elastic joint is a defining trait of all whales that sieve their food from the water, including the blue, humpback and right whales. Not all of them are lunge-feeders. The right and bowhead whales swim through schools of plankton with jaws open, while gray whales suck up mud from the seafloor. But all of them rely on the elastic joint that allows them to open their mouths as wide as possible. It was an important innovation that allowed this group – the mysticetes, or baleen whales – to hunt the smallest of prey in the largest of volumes. It allowed them to evolve into the largest animals this planet has ever seen.

But that wasn’t always the case. Ancient whales had the same fused jaws found in toothed whales like sperm whale and dolphins (and, for that matter, humans). Now, Erich Fitzgerald from Museum Victoria in Melbourne has found a fossil that bridges the gap between this standard set-up and the flexible jaws of titans.

The animal in question is called Janjucetus hunderi, named after Jan Juc Beach where it was found, the Greek word for whale (‘cetus’) and Staumm Hunder, the teenage surfer who found the fossil. The bones were found in the late 1990s but they languished in obscurity under Fitzgerald started working on them in earnest in 2003. Janjucetus lived 25 million years ago in the seas of Australia, and was around the size of a modern bottlenose dolphin.

Its remains include a complete skull. The lower jaw is distinctly unlike a mysticete’s – it had many teeth, and the two halves were clearly fused. But the upper jaws are classic mysticete – they were extremely wide. Coupled with its short, blunt snout, this animal had a very big mouth for its size.

Based on Janjucetus,Fitzgerald thinks that the early mysticetes fed in a different way than their modern descendants. They didn’t have the adaptations needed to swim into a large swarm of plankton and sieve them from the water. Instead, they were probably suction-feeders. They would swim up, pull back their tongues and open their large jaws, creating a sudden flow of water that would sweep any nearby animals into their mouths. They probably took one or two fish or squid at a time.

The wide upper jaw of Janjucetus, along with its short blunt snout, would have given it a cavernous mouth that optimised the suction it could create. Only later did the mysticetes develop adaptations that allowed their mouths, and thus their bodies, to become really big. Before they evolved to sieve, they sucked and sucked hard.

Comments (1)

I seem to recall the original description for Janjucetus suggesting that it was a predator in the mode of sea lions. That was 2006–and also by Fitzgerald. I wonder what made him change his mind? Suction feeding would be a logical precursor to baleen feeding, but Janjucetus seems a little more…”go in for the kill” than suction-feeder.

Now, a whale like Aetiocetus speaks more to the suction feeding angle, doesn’t it? It had teeth but it also had short baleen plates.